Amnesty International has accused the Nigerian military and civilian militias of "extensive human rights violations" in their fight against extremists in the northeast of the country.

The global rights watchdog said gruesome video footage, images and witness testimonies collected during a research mission in Borno state found "fresh evidence of extrajudicial executions and serious human rights violations" in the region.

The footage includes detainees having their throats cut one by one and then dumped in mass graves by men who "appear to be members of the Nigerian military and the Civilian Joint Task Force (CJTF)", it said.

Salil Shetty, secretary general of Amnesty International, said: "The ghastly images are backed up by the numerous testimonies we have gathered which suggest that extrajudicial executions are, in fact, regularly carried out by the Nigerian military and CJTF.

"These are not the images we expect from a government which sees itself as having a leadership role in Africa."

The video also shows the aftermath of a Boko Haram raid on a village, in which nearly 100 people were killed by the militants and scores of houses and buildings destroyed.

More than 4,000 people have been killed this year in the conflict between Boko Haram and the Nigerian military. More than 600 of these were extrajudicially executed, Amnesty said.

In April, Boko Haram abducted 276 schoolgirls from Chibok in northeastern Nigeria. Some have managed to escape, but the militants are still holding 219 captive.

Boko Haram has staged attacks on villages in the remote northeast, where it aims to create a strict Islamic state.

Amnesty has called on Nigerian authorities to stop human rights violations in the military and urged it to hold investigations into the most serious allegations.

In a statement, Nigeria's defence ministry said it took the allegations "very seriously".

It said: "Much as the scenes depicted in these videos are alien to our operations and doctrines, (they have) to be investigated to ensure that such practices have not crept surreptitiously into the system."